Manufacturing method of aluminum-based alloy plated steel

ABSTRACT

The present disclosure relates to an aluminum-based plated steel that is provided for vehicles by hot forming, an aluminum-based alloy plated steel manufactured using the same, and method of manufacturing thereof.

The present application is a divisional of U.S. patent application Ser.No. 16/470,723, filed on Jun. 18, 2019, which is a 371 application ofInternational Patent Application No. PCT/KR2017/015295, filed on Dec.21, 2017, which claims benefit of priority to Korean Patent ApplicationNo. 10-2016-0178532 filed on Dec. 23, 2016, disclosures relate to analuminum-based plated steel that is provided for vehicles by hotforming, an aluminum-based alloy plated steel manufactured using thesame, and method of manufacturing thereof.

TECHNICAL FIELD Background Art

Recently, in the manufacturing of vehicle components requiring corrosionresistance and crashworthiness, such as vehicle structural members,reinforcing members, and the like, a hot forming (or referred to as “hotpress forming”) technique has been increasingly used.

The hot press forming technique has been known as an excellent techniquefor securing ultra-high strength for reducing a weight of a vehicle andimproving crashworthiness which have been required of automobilecompanies. The hot press forming technique may resolve the problemsrelated to formability, shape fixability, and the like, during a coldpress forming, caused by ultra-high strength of steel.

Also, patents and techniques relating to an aluminum plated or hot-dipgalvanized steel have been developed to secure corrosion resistance of avehicle component. Reference 1 discloses a technique relating to amethod of manufacturing an aluminum plated steel for hot press forming.

In Reference 1, steel for hot press forming (HPF) may have low strengthbefore a heat treatment. A high temperature heating may be performed inthe HPF process, and a rapid cooling process may be performed by cooinga mold, thereby manufacturing a hot press formed component havingmartensite as a main phase in a final component. Also, an Fe—Alintermetallic compound may be formed in a plating layer during a heattreatment such that heat resistance and corrosion resistance may besecured.

When an Al-plated steel is heated to approximately 900° C. in a heatingfurnace, various intermetallic compounds such as FeAl, Fe2Al5, and thelike, may be formed in an Al-plated layer, and an alloy layer may beformed. Such intermetallic compounds may have high brittleness, suchthat the intermetallic compounds may be separated from a plating layerduring a press forming process and may be adhered to a press surface.Accordingly, it may be difficult to perform a continuous press formingprocess, which may be disadvantageous, and there may be a problem ofdegradation of corrosion resistance.

Thus, there has been increasing demand for a steel for hot press formingwhich may secure excellent corrosion resistance even after hot pressforming, and a hot press formed member using the same.

PRIOR ART

-   (Reference 1) U.S. Pat. No. 6,296,805

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide an aluminum-basedplated steel having improved corrosion resistance before and after hotpress forming, an aluminum-based alloy plated steel material using thesame, and a method of manufacturing the aluminum-based plated steelmaterial and the aluminum-based alloy plated steel material.

However, aspects of the present disclosure are not limited thereto.Additional aspects will be set forth in part in the description whichfollows, and will be apparent from the description to those of ordinaryskill in the related art.

Technical Solution

According to an aspect of the present disclosure, an aluminum-basedplated steel having excellent corrosion resistance is provided, thealuminum-based plated steel including base steel comprising, by wt %,0.18 to 0.25% of C, 0.1 to 0.5% of Si, 0.9 to 1.5% of Mn, 0.03% or lessof P, 0.01% or less of S, 0.01 to 0.05% of Al, 0.01 to 0.5% of Cr, 0.01to 0.05% of Ti, 0.001 to 0.005% of B, 0.009% or less of N, and a balanceof Fe and inevitable impurities, and the aluminum-based plated steelfurther comprises an Al-based plated layer formed on a surface of thebase steel, the Al-based plated layer comprises an Al—Si crystallizedphase, and an average gain size of the Al—Si crystallized phase is 4 μmor less.

According to another aspect of the present disclosure, a method ofmanufacturing an aluminum-based plated steel having excellent corrosionresistance is provided, the method including preparing a base steelcomprising, by wt %, 0.018 to 0.25% of C, 0.1 to 0.5% of Si, 0.9 to 1.5%of Mn, 0.03% or less of P, 0.01% or less of S, 0.01 to 0.05% of Al, 0.05to 0.5% of Cr, 0.01 to 0.05% of Ti, 0.001 to 0.005% of B, 0.009% or lessof N, and a balance of Fe and inevitable impurities; plating theprepared base steel by submerging the base steel in a hot-dip aluminumplating bath; and cooling the plated base steel at 3 to 25° C./s.

According to another aspect of the present disclosure, an aluminum-basedalloy plated steel having excellent corrosion resistance is provided,the aluminum-based alloy plated steel including base steel comprising,by wt %, 0.018 to 0.25% of C, 0.1 to 0.5% of Si, 0.9 to 1.5% of Mn,0.03% or less of P, 0.01% or less of S, 0.01 to 0.05% of Al, 0.05 to0.5% of Cr, 0.01 to 0.05% of Ti, 0.001 to 0.005% of B, 0.009% or less ofN, and a balance of Fe and inevitable impurities, and the aluminum-basedalloy plated steel further comprises an Fe—Al alloy plated layer formedby a heat treatment, and the Fe—Al alloy plated layer comprises a middlelayer concentrated with Si, and an average grain size of the middlelayer is 2 μm or less.

According to another aspect of the present disclosure, a method ofmanufacturing an aluminum-based alloy plated steel having excellentcorrosion resistance is provided, the method including preparing a basesteel comprising, by wt %, 0.018 to 0.25% of C, 0.1 to 0.5% of Si, 0.9to 1.5% of Mn, 0.03% or less of P, 0.01% or less of S, 0.01 to 0.05% ofAl, 0.05 to 0.5% of Cr, 0.01 to 0.05% of Ti, 0.001 to 0.005% of B,0.009% or less of N, and a balance of Fe and inevitable impurities;plating the prepared base steel by submerging the base steel in ahot-dip aluminum plating bath; cooling the plated based steel at 3 to25° C./s and manufacturing an aluminum-based plated steel having anAl-based plated layer; heating the aluminum-based plated steel at 800 to1000° C. for 3 to 20 minutes, where a change of the heating rate at atemperature range of 600 to 700° C. is 0.05° C./s² or less in absolutevalue; and rapidly cooling the heated aluminum-based plated steel.

Advantageous Effects

According to an aspect of the present disclosure, an aluminum-basedplated steel having excellent corrosion resistance and also analuminum-based alloy plated steel having ultra-high strength of 1300 MPaor higher and having improved corrosion resistance after hot pressforming may be provided, and an appropriate method of manufacturing theabove-described steel may be provided, which are technical effects ofthe present disclosure.

DESCRIPTION OF DRAWINGS

FIG. 1 is an image obtained by observing plating layers of inventivematerial A and comparative material C among embodiments of the presentdisclosure;

FIG. 2 is an image obtained by observing a cross-sectional surface of anFe—Al alloy plated layer of embodiment A-1 among embodiments of thepresent disclosure; and

FIG. 3 is an image obtained by observing a middle layer portion in theplating layer in FIG. 2 .

BEST MODE FOR INVENTION

Hereinafter, preferable example embodiments of the present disclosurewill be described in detail.

The inventors have been found that, when a hot press forming process isperformed using a steel used for hot press forming, properties of aplating layer, particularly an aluminum or aluminum alloy plated layer(hereinafter, referred to as “aluminum-based plated layer” or “Al-basedplated layer”), before a heat treatment, may greatly affect corrosionresistance. They also have found that, property of an alloyed platedlayer (hereinafter, referred to as “aluminum-based alloy plated layer”)after a hot press forming heat treatment may be important with respectto improvement of corrosion resistance of a hot press formed member.

Accordingly, to improve corrosion resistance, the inventors conductedresearches on a plating layer of the aluminum-based plated steel and analloy plated layer of an aluminum-based alloy plated steel, alloyedafter a hot press forming heat treatment, and reached to the presentdisclosure.

An aluminum-based plated steel will be described in detail.

The aluminum-based plated steel may include base steel and an Al-basedplated layer formed on a surface of the base steel.

The base steel may include, by wt %, 0.18 to 0.25% of C, 0.1 to 0.5% ofSi, 0.9 to 1.5% of Mn, 0.03% or less of P, 0.01% or less of S, 0.01 to0.05% of Al, 0.05 to 0.5% of Cr, 0.01 to 0.05% of Ti, 0.001 to 0.005% ofB, 0.009% or less of N, and a balance of Fe and inevitable impurities.In the description below, a compositional range of the base steel willbe described in detail (hereinafter, wt %).

Carbon (C): 0.18 to 0.25%

C is an essential element which may improve strength of martensite. Whena content of C is less than 0.18%, it may be difficult to obtainsufficient strength for securing crashworthiness. When a content of Cexceeds 0.25%, impact toughness of a slab may degrade, and weldabilityof an HPF formed member may degrade. Thus, in the present disclosure, itmay be preferable to control a content of C to be 0.18 to 0.25%.

Silicon (Si): 0.1 to 0.5%

Si may be added for deoxidation of steel, and may be effective forhomogenization of a structure of a member after an HPF process. When acontent of Si is less than 0.1%, an effect of deoxidation and structurehomogenization may not be sufficiently obtained. When a content of Siexceeds 0.5%, it may be difficult to secure good quality of a hot-dipaluminum plated surface due to a Si oxide formed on a surface of a steelsheet during annealing. Thus, it may be preferable to control a contentof Si to be 0.5% or less.

Mn: 0.9 to 1.5%

Mn may be added to secure hardenability of steel similarly to Cr, B, andthe like. When a content of Mn is less than 0.9%, it may be difficult tosecure sufficient hardenability such that bainite may be formed, and itmay be difficult to secure sufficient strength. Also, when a content ofMn exceeds 1.5%, costs for manufacturing a steel sheet may increase, andMn may be segregated in a steel material such that bendability of an HPFformed member may significantly degrade. Thus, in the presentdisclosure, it may be preferable to control a content of Mn to be 0.9 to1.5%.

P: 0.03% or less (excluding 0%)

P is a grain boundary segregation element which may degrade variousproperties of a hot press formed member. Thus, it may be preferable toadd as a low a content of P as possible. When a content of P exceeds0.03%, bendability of a formed member, impact properties, weldability,and the like, may degrade. Thus, it may be preferable to control anupper limit content of P to be 0.03%.

S: 0.01% or less (excluding 0%)

S is an element which may be present in steel as an impurity, and maydegrade the bendability and weldability of a formed member. Thus, it maybe preferable to add as low a content of P as possible. When a contentof S exceeds 0.01%, bendability and weldability of a formed member maydegrade. Thus, it may be preferable to control an upper limit content ofS to be 0.01%.

Al: 0.01 to 0.05%

Al may be added for the purpose of deoxidation in the steel makingsimilarly to Si. To achieve the purpose, 0.01% or higher of Al may needto be added. When a content of Al exceeds 0.05%, the effect of additionof Al may be saturated, and surface quality of a plated material may bedeteriorated. Thus, it may be preferable to control an upper limitcontent of Al to be 0.05%.

Cr: 0.01 to 0.5%

Cr may be added to secure hardenability of steel similarly to Mn, B, andthe like. When a content of Cr is less than 0.01%, it may be difficultto secure sufficient hardenability. When a content of Cr exceeds 0.5%,hardenability may be sufficiently secured, but properties thereof may besaturated, and costs for manufacturing steel may increase. Thus, in thepresent invention, it may be preferable to control a content of Cr to be0.01 to 0.5%.

Ti: 0.01 to 0.05%

Ti may create TiN by being combined with nitrogen remaining asimpurities in steel, and may thus be added to cause solid solution B toremain, solid solution B which may be essential for securinghardenability. When a content of Ti is less than 0.01%, it may bedifficult to obtain the effect of addition of Ti. When a content of Tiexceeds 0.05%, the properties thereof may be saturated, and costs formanufacturing steel may increase. Thus, in the present disclosure, itmay be preferable to control a content of Ti to be 0.01 to 0.05%.

B: 0.001 to 0.005%

B may be added to secure hardenability in a hot press formed membersimilarly to Mn and Cr. To achieve the purpose described above, 0.001%or higher of B may need to be added. When a content of B exceeds 0.005%,the effect of the addition of B may be saturated, and hot press formingproperties may significantly degrade. Thus, in the present disclosure,it may be preferable to control a content of B to be 0.001 to 0.005%.

N: 0.009% or less

M may be present as an impurity in steel, and it may be preferable to aslow a content of N as possible. When a content of N exceeds 0.009%, asurface defect may occur in steel. Thus, it may be preferable to controlan upper limit content of N to be 0.009%.

As a remainder, Fe and inevitable impurities may be included. However,addition of other alloy elements may not be excluded without departingfrom the technical idea of the present disclosure. To obtain a morepreferable effect, elements as below may be additionally included.

One or more of molybdenum (Mo) and tungsten (W): 0.001 to 0.5%

Mo and W are elements which may improve hardenability and may strengthenprecipitation, and may be effective for securing high strength. When acontent of Mo and W is less than 0.001%, the effect of securingsufficient hardenability and strengthened precipitation may not beobtained. When a content Mo and W exceeds 0.5%, the effect of additionof Mo and W may be saturated, and manufacturing costs may increase.Thus, in the present disclosure, it may be preferable to control acontent of Mo and W to be 0.001 to 0.5%.

A combination of one or more of niobium (Nb), zirconium (Zr), orvanadium (V): 0.001 to 0.4%

Nb, Zr, and V are elements which may increase strength of a steel sheet,may refine grains and may improve heat treatment properties. When acontent of one or more of Nb, Zr, and V is less than 0.001%, it may bedifficult to obtain the above-described effect. When the content exceeds0.4%, manufacturing costs may significantly increase. Thus, in thepresent disclosure, a content of the elements may be controlled to be0.001 to 0.4%.

One or more of copper (Cu) and nickel (Ni): 0.005 to 2.0%

Cu is an element which may improve strength by forming fine Cuprecipitation, and Ni is an element which may be effective for improvingstrength and heat treatment properties. When a content of the elementsis less than 0.005%, sufficient strength may not be obtained. When thecontent exceeds 2.0%, workability may be deteriorated, and manufacturingcosts may increase. Thus, in the present disclosure, a preferablecontent of Cu and Ni may be 0.005 to 2.0%.

One or more of antimony (Sb), tin (Sn), or bismuth (Bi): 0.03% or less.

Sb, Sn, and Bi are grain boundary segregation elements which may beconcentrated on an interfacial surface between a plating layer and baseiron during an HPF heating process such that adhesive properties of theplating layer may improve. By improving adhesive properties of theplating layer, the element may prevent the plating layer from beingseparated during a hot press forming. As Sb, Sn, and Bi may have similarcharacteristics, the three elements may be mixed and the mixture may beused, and in this case, it may be preferable to control the combinationof one or more of the elements to be 0.03%. When the combination of theelements exceeds 0.03%, brittleness of base steel may be deterioratedduring a hot press forming process.

In the aluminum-based plated steel, an Al-based plated layer may beformed on a surface of the base steel. The Al-based plated layer mayinclude an Al—Si crystallized phase.

The Al—Si crystallized phase may refer to a phase formed bycrystallization in a liquid state in the Al-based plating layer. As anexample, FIG. 1 is an image of an Al—Si crystallized phase (an arrow inFIG. 1 ), and the Al—Si crystallized phase may have a spherical shape ora needle shape depending on the form of the phase.

A preferable size (an average equivalent circular grain size) of theAl—Si crystallized phase may be 4 μm or less. Also, a preferable aspectratio of the Al—Si crystallized phase may be 10 or less. A preferablecontent of Si in the Al—Si crystallized phase may be 3 to 25 wt %. Asize, an aspect ratio, and a content of Si of the Al—Si crystallizedphase may be factors which may affect the complete remelting of theAl—Si crystallized phase during a subsequent hot press forming process.Also, when Si of the crystallized phase is fully solute in an Fe—Almiddle layer formed in an Fe—Al alloy plated layer of the aluminum-basedalloy plated steel, grain growth of the middle layer may be preventedconsequently. Also, the finer the grain size (preferably, 2 μm or less),propagation of cracks which may be inevitably created during a hot pressforming process may further be prevented in the middle layer such thatcorrosion resistance may improve.

When a size (an average equivalent circular grain size) of the Al—Sicrystallized phase exceeds 4 μm, it may be difficult for thecrystallized phase to be remelted during a hot press forming process.Even through a size of the Al—Si crystallized phase is 4 μm or less,when an aspect ratio exceeds 10, it may be difficult for thecrystallized phase to be completely remelted in a long shaft directionsuch that there may be a limitation in obtaining the above-describedeffect. When a content of Si in the crystallized phase is less than 3 wt%, it may be difficult to prevent the grain growth of the middle layer.When the content of Si exceeds 25 wt %, excessive content of Si may needto be added to a plating bath, which may significantly increase amelting point of the plating bath such that plating workability may bedeteriorated.

A preferable thickness of the Al-based plated layer may be 15 to 35 μm.When a thickness of the Al-based plated layer is less than 15 μm, it maybe difficult to secure sufficient corrosion resistance in thealuminum-based alloy plated steel. When the thickness exceeds 35 μm, athickness of the plating layer in the final aluminum-based alloy platedsteel may excessively grow such that workability may be deteriorated.

In the description below, a method of manufacturing an aluminum-basedplated steel of the present disclosure will be described.

In the present disclosure, base steel having the above-describedcomposition may be prepared, and it may be preferable to manufacture thealuminum-based plated steel by performing a hot-dip aluminum platingprocess.

The base steel may be a hot-rolled steel sheet, a cold-rolled steelsheet, an annealed steel sheet, and the like, and a type of the basesteel may not be particularly limited. Any steel material which may beable to be applied in the technical field of the present disclosure maybe used.

In the hot-dip aluminum plating process, the base steel may be submergedin a hot-dip plating bath, an amount of plating to be attached may beadjusted using an air knife, a cooling rate may be adjusted, and a sizeand an aspect ratio of the Al—Si crystallized phase in the Al-basedplated layer may be adjusted.

A preferable cooling rate after the plating may be 3 to 25° C./s. Whenthe cooling rate is less than 3° C./s, the Al—Si crystallized phase maybecome excessively coarse, and the solidification on a top roll may beincomplete such that quality of plating surface may be deteriorated.When the cooling rate exceeds 25° C./s, the Al—Si crystallized phase maybe distributed in a fine and uniform manner, but costs may increase dueto excessive investment to cooling facility.

A preferable composition of the hot-dip aluminum plating bath mayinclude, by wt %, 8 to 11% of Si, 3% or less of Fe, and a balance of Aland inevitable impurities.

In the description below, an aluminum-based alloy plated steel will bedescribed in detail.

The aluminum-based alloy plated steel may include a Fe—Al alloy platedlayer obtained by alloying the Al-based plated layer with an elementsuch as Fe in the base steel on a surface of the base steel having theabove-described composition. The Fe—Al alloy plated layer may include amiddle layer concentrated with Si.

As illustrated in FIG. 2 , the aluminum-based alloy plated steel mayinclude a diffusion layer, an intermetallic compound layer, a middlelayer, an outermost layer, and the like, from the base steel. The middlelayer may refer to a layer concentrated with and comprising Si andcontinuously or discontinuously formed.

The middle layer may occupy 5 to 30% based on a thickness of across-sectional surface of the Fe—Al alloy plated layer. When athickness of the middle layer occupies less than 5% based on a thicknessof a cross-sectional surface of the Fe—Al alloy plated layer, it may beinsufficient to prevent propagation of cracks in a plating layer. Whenthe thickness exceeds 30%, a heat treatment may need to be excessivelyperformed on a steel blank at a significantly high temperature and for along period of time, which may degrade productivity.

A preferable size of the middle layer may be 2 μm or less. The middlelayer may prevent propagation of cracks, and to this ends, grains mayneed to be fine. When a grain size of the middle layer exceeds 2 μm, itmay be difficult to secure the sufficient effect of preventingpropagation of cracks.

A preferable content of Si of the middle layer may be 7 to 14 wt %. Siof the middle layer may stabilize a phase of the middle layer and mayalso be important for preventing grain growth. Thus, when a content ofSi is less than 7 wt %, it may be difficult to obtain theabove-described effect, and to include Si of higher than 14%, anexcessive content of Si may need to be added in a plating bath, a heattreatment may need to be performed on a blank at a significantly hightemperature for a long period of time, a diffusion layer may becomeexcessively thick (exceeding 16 μm) such that weldability may degrade.

The base steel of the aluminum-based alloy plated steel of the presentdisclosure may include a martensite structure of 95% or higherpreferably, and a preferable tensile strength may be 1300 MPa or higher.

In the description below, a method of manufacturing an aluminum-basedalloy plated steel will be described in detail.

The aluminum-based alloy plated steel may be manufactured by hot-pressheating the above-described aluminum-based plated steel and cooling theheated aluminum-based plated steel, and a forming process may beperformed after the heating.

The aluminum-based plated steel may be prepared, and a heat treatment inwhich the aluminum-based plated steel is heated at 800 to 1000° C. for 3to 20 minutes may be performed.

When the heating temperature is less than 800° C., sufficient austenitemay not be obtained such that it may be difficult to secure sufficientstrength even when a rapid cooling is performed. When the heatingtemperature exceeds 1000° C., heating costs may be excessive, andheating facility may be deteriorated when the facility is used for along time. When the heating time is less than 3 minutes, homogenizationof an alloy element such as carbon and manganese in the base steel maybe difficult. When the heating time exceeds 20 minutes, a thickness ofthe diffusion layer may excessively increase such that weldability maydegrade.

A preferable heating rate during the heat treatment may be 1 to 10°C./s. When the heating rate is less than 1° C./s, it may be difficult tosecure productivity of the aluminum-based alloy plated steel material.When the heating rate exceeds 10° C./s, it may be difficult for theAl—Si crystallized phase to be fully remelted in the plating layer, andexcessive costs may be required to increase the heating rate, which maynot be preferable.

A preferable change of the heating rate at a temperature range of 600 to700° C. during the heating may be 0.05° C./s2 or less in absolute value.In the temperature range of 600 to 700° C. during the heating, the Alplating may be performed and the Al—Si crystallized phase may beremelted. When a change of the heating rate exceeds 0.05° C./s2, theAl—Si crystallized phase may not be stably and smoothly redistributed onthe middle layer of the plating layer, and accordingly, it may bedifficult to secure a desired grain size, a desired content of Si, adesired thickness of the middle layer.

The steel heated as above may be rapidly cooled in a mold. The steel maybe cooled to 300° C. or lower, and a preferable cooling rate during thecooling may be 20 to 200° C./s. When the cooling rate is less than 20°C./s, a cooling effect may not be obtained. When the cooling rateexceeds 200° C./s, an effect of martensite transformation by hot pressforming may decrease due to excessive cooling.

MODE FOR INVENTION

In the description below, an example embodiment of the presentdisclosure will be described in greater detail. It should be noted thatthe exemplary embodiments are provided to describe the presentdisclosure in greater detail, and to not limit the present disclosure.

Embodiment 1

A cold-rolled steel sheet having a composition (wt %, and a remainderwas Fe and inevitable impurities) as in Table 1 was prepared, and aplating process was performed under Al plating conditions indicated inTable 2.

A size of an Al—Si crystallized phase, a content of Si of the Al—Sicrystallized phase, and an aspect ratio of the Al—Si crystallized phaseof the plating layer of the aluminum-based plated steel manufactured asabove were observed, and the results were indicated in Table 3. Also,corrosion resistance of the Al plated steel was examined, and the resultwas indicated in Table 3 as well.

A size, an aspect ratio, and a content of Si of the Al—Si crystallizedphase on the plating layer were obtained by observing three regions of across-sectional surface of the plating layer using a scanning electronmicroscope (SEM), and by carrying out an image analysis and by using anEDS, and the averages were listed in the table.

TABLE 1 C Si Mn P S Al Cr Ti B N 0.21 0.25 1.2 0.013 0.003 0.02 0.150.03 0.0023 0.0035

TABLE 2 Composition of Al-based Amount Plating Bath of Cooling (wt %, aremainder Attached Rate after of Al) Plating SolidificationClassification Si Fe (g/m²) (° C./s) Note Condition 1 8.3 2.1 75 8.9Inventive Material A Condition 2 8.7 1.9 82 4.5 Inventive Material BCondition 3 7.4 2.0 78 1.8 Comparative Material C Condition 4 5.2 1.8 6231.2 Comparative Material D

TABLE 3 Size of Content of Aspect Ratio of Al—Si Si of Al—Si Al—SiCrystallized Crystallized Crystallized Classification Phase (μm) Phase(wt. %) Phase Inventive 2.0 11.5 3.8 Material A Inventive 1.4 6.0 3.0Material B Comparative 6.0 18.2 15.6 Material C Comparative 0.9 2.7 2.8Material D

As indicated in Table 3, a size, a content of Si, and an aspect ratio ofthe Al—Si crystallized phase of the inventive material were included inthe range of the present disclosure, whereas the comparative materialdid not satisfy the range of the present disclosure.

Particularly, (a) and (b) of FIG. 1 are images obtained by observingplating layers of inventive material A and comparative material C,respectively. In inventive material A, the Al—Si crystallized phase wasformed, whereas in comparative material C, a very sharp crystallizedphase was formed.

Embodiment 2

The aluminum-based plated steel material manufactured through Embodiment1 above was prepared, and a heat treatment was performed underconditions indicated in Table 4 below.

After performing the heat treatment, an alloy layer formed on a surfaceof a manufactured aluminum-based alloy plated steel was analyzed, andthe result was listed in Table 5. Specifically, a grain size and acontent of Si of a middle layer formed in the alloy layer were measured,and the results were listed in Table 5.

As for a corrosion resistance test, a phosphate process and a paintingprocess were performed on the member sample under a GMW14872 method, anda maximum blister width was measured after 53 cycles for an X-cut sampleunder a CCT condition.

A thickness of the middle layer was obtained by calculating a ratio of athickness of the middle layer to a thickness of an overall Fe—Al alloyplated layer using an optical microscope. A content of Si and a size ofthe middle layer were obtained by processing a cross-sectional surfaceusing a focused ion beam (FIB), and by analyzing a grain size and acomposition using a transmission electron microscope (TEM).

TABLE 4 Heating Change Al-based Temper- Heating Heating of Heating Steelature Time Rate Rate Material (° C.) (minutes) (° C./s) (° C./s²)Classification Inventive 900 6 2.8 −0.01 Inventive Material A ExampleA-1 Inventive 900 6 2.3 −0.07 Comparative Material A Example A-2Inventive 900 6 0.5 −0.002 Comparative Material A Example A-3 Inventive900 6 15.2 −0.004 Comparative Material A Example A-4 Inventive 950 4 3.5−0.02 Inventive Material B Example B-1 Inventive 760 10 2.1 −0.008Comparative Material B Example B-2 Inventive 950 30 4.6 −0.007Comparative Material B Example B-3 Comparative 930 6 4.1 −0.01Comparative Material C Example C-1 Comparative 930 6 3.8 −0.013Comparative Material D Example D-1

TABLE 5 Grain Content Ratio of Corrosion Size of of Si of ThicknessResistance Middle Middle of Middle Test Tensile Layer Layer Layer(blister Strength Classification (μm) (wt. %) (%) width, mm) (MPa)Inventive 0.9 8.8 10.4 0.5 1506 Example A-1 Comparative 2.3 5.8 8.3 1.51487 Example A-2 Comparative 0.7 10.5 14.6 0.5 1527 Example A-3Comparative 2.1 6.2 4.8 1.5 1498 Example A-4 Inventive 0.8 8.7 12.5 0.51513 Example B-1 Comparative 0.4 7.9 3.2 2.0 1051 Example B-2Comparative 1.7 11.6 21.4 0.5 1473 Example B-3 Comparative 3.5 5.4 11.82.5 1537 Example C-1 Comparative 4.1 4.2 13.8 2.5 1516 Example D-1

As indicated in the results in Table 5 above, both of inventive examplesA-1 and B-1 satisfying the conditions of the present disclosure hadexcellent corrosion resistance, and secured high strength. Comparativeexample A-2 was the case in which a change value of a temperature risingspeed of the present disclosure was not satisfied, and a grain size ofthe middle layer was coarse, and corrosion resistance degraded.

In comparative example A-3, as a temperature rising speed was too low tosecure high productivity, and was thus be classified as a comparativeexample. In comparative example A-4, as a temperature rising speed wastoo fast such that a grain size of the middle layer was coarse, andcorrosion resistance degraded.

In comparative example B-2, as a heat treatment temperature was low,sufficient strength was not secured. In comparative example B-3, a heattreatment was performed for a long time, and accordingly, productivitywas low, and weldability was able to be deteriorated, and thus,comparative example B-3 was classified as a comparative example.

In comparative examples C-1 and C-2, a hot press forming heat treatmentprocess satisfied the range of the present disclosure, but an Al platedsteel material which did not satisfied the range of the presentdisclosure was used. Accordingly, a grain size of the middle layer wascoarse, and corrosion resistance degraded.

FIG. 2 is an image obtained by observing a hot press formed membersample of inventive example A-1, and FIG. 3 is an image obtained byobserving the middle layer in FIG. 2 and the result obtained byanalyzing the composition thereof. As indicated in FIG. 2 , the middlelayer occupied 5 to 30% in the overall Fe—Al alloy plated layer, and asindicated in FIG. 3 , an average grain size of the middle layer was 2 μmor less, and a content of Si was 7 to 14 wt % (see a box region in thetable in FIG. 3 ). Nos. 1, 2, and 23 in the table in FIG. 3 indicatedthat a phase of an outermost layer forming a boundary with the middlelayer or a phase of an intermetallic compound layer was observed.

The invention claimed is:
 1. A method of manufacturing an aluminum-basedalloy plated steel having excellent corrosion resistance, comprising:preparing a base steel comprising, by wt %, 0.18 to 0.25% of C, 0.1 to0.5% of Si, 0.9 to 1.5% of Mn, 0.03% or less of P, 0.01% or less of S,0.01 to 0.05% of Al, 0.01 to 0.5% of Cr, 0.01 to 0.05% of Ti, 0.001 to0.005% of B, 0.009% or less of N, and a balance of Fe and inevitableimpurities; plating the prepared base steel by submerging the base steelin a hot-dip aluminum plating bath; cooling the plated base steel at 3to 25° C./s and manufacturing an aluminum-based plated steel having anAl-based plated layer; heating the aluminum-based plated steel at 800 to1000° C. for 3 to 20 minutes, wherein a change of the heating rate at atemperature range of 600 to 700° C. is 0.05° C./s² or less in absolutevalue; and rapidly cooling the heated aluminum-based plated steel,wherein the rapidly cooling is performed to 300° C. or lower at acooling rate of 20 to 200° C./s.
 2. The method of claim 1, wherein aheating rate during the heating is 1 to 10° C./s.
 3. The method of claim1, further comprising: forming the heated aluminum-based plated steel.